Today we're looking at a new book by Maxim Popenker and Anthony G. Williams, Sub-Machine Gun. It should come as no surprise that a collaboration between these two folks results in an excellent reference book (we previously reviewed their work on WWI aircraft guns). This book includes both a detailed section on submachine gun cartridges, and the guns themselves - organized by nationality and focusing largely on experimental and developmental models. It's an excellent reference work, and a valuable addition to a good gun library.